A fiber-wrapped composite gas cylinder for high-pressures, also known as a filament-wound pressure vessel, essentially consists of two components: a cylindrical or spherical containment shell or liner (hereinafter primarily referred to as a “liner”) and a structural composite material. The composite is a continuously wrapped fiber-resin matrix material. The liner, over which the composite is wrapped and cured in place, provides means for preventing or minimizing gas permeation through the composite. In addition, in composite cylinders with a metallic liner, the liner provides a means for load sharing with the composite.
Machine filament-winding over a liner for a pressure vessel is well known and exemplified by the present inventor's 1969 India patent (Specification 111958) for a filament winding machine. The present invention utilizes prior art for filament winding and involves only an improved and easy to manufacture liner used in a composite gas cylinder for high-pressures.
For most consumer type composite pressure vessels, generally known as compressed gas cylinders, or pressure receptacles, the metal liner is typically made of aluminum or steel. Other metals including alloys thereof have been used. Examples of other metals known for use in liners in general are stainless steel, nickel, nickel alloys, titanium, and titanium alloys. Historically steel and aluminum alloys are metals used for seamless cylinders for the storage and transportation of compressed gases. Aluminum alloy is the principal metal discussed herein because of its strength and weight characteristics make it a preferred metal of the seamless liner of the invention. However, the term metal in this disclosure is intended to be defined broadly and is not limited to aluminum alloy.
The reason composite cylinders are preferred over solid metal cylinders is that for higher pressures, the required wall thickness of the solid metal cylinder would be such that the payload fraction becomes uneconomical for transportation and for application as a fireman's back-pack and in space craft. When used for natural gas fuel tanks, metal cylinders are prone to corrosion and the risk of catastrophic failure is a serious safety issue. Failure probability and consequences tend to be unacceptable to the society.
Weight and safety issues are critical in applications for natural gas or hydrogen fuel tanks, especially for Alternate Fuel Vehicles. Over the past three decades, compressed gas cylinders made of fiber-wrapped metallic and non-metallic liners have been shown to be a solution for overcoming the weight limitation issues as well as offering improved level of safety because of corrosion resistance and a benign failure mode.
However, a fiber-wrapped composite cylinder for high-pressure compressed gas storage and transportation is costly and complicated to design, test, and manufacture in compliance with regulatory standards. The liner is a significant cost factor in the cost of the cylinder and this invention provides a means to lower costs and simplify manufacture of the liner.
Aluminum alloy liners for fiber-wrapped composite cylinders are typically manufactured from a sheet, a billet, or flow formed seamless tubing. The aluminum alloy is usually a 6000 series aluminum alloy, for example, AA6061-T6 aluminum alloy. Liners may also be made of steel and other metals with a high strength to density ratio.
In manufacturing a liner, if the starting liner material is cast billet, the billet is shaped like a log and is pre-inspected to assure that it is free from harmful defects the cast billet is placed on a conveyor belt and cut to the desired size by an automated saw. The sawn piece is called a slug and is almost the same weight and diameter as the finished product. The slug is then placed inside a die in a backward extrusion press. The press forces a punch against the slug. The metal of the slug flows backwards around the punch forming a large, hollow, cup-shaped product is what will be shaped into a liner.
The extruded cup-shape form is further drawn on a mandrel to required length and with thickened wall segment at the open end. The thickened wall segment at the open end incorporates added material to assure that the formed dome has thickness distribution to minimize stresses along the dome contour and through thickness. The thickened wall segment at each end of the tube also permits a mechanical spinning process on the thickened wall segment to form a concave end contour.
The extruded cup-shaped liner with the thickened wall segment is put through a process called heading, necking or swaging. The open end of this liner is heated and forced into a closing die to close the open end of the cup around a boss, also called a fitting. Alternatively, the open end, or both ends if tubing is used, is formed into a head by computer numerical control (CNC) mechanical spinning to a very precise contour with an extended cylindrical neck region for accommodating a boss, which is often a nozzle, flange, threaded connection, or a solid plug. The threaded connection is to permit assembly of valving, pressure regulators and safety devices. The aluminum alloy liner is then subjected to solution heat-treatment to bring the liner mechanical properties to optimum strength level. At that point, the seamless liner is finished and ready for over wrapping with composite.
For long cylinders, the standard method to make the liner is to start with a thick walled short tube, subjected to flow forming process. The short tube is fitted over a steel mandrel supported between head stock and a tail stock and made to rotate together. CNC controlled compression forces are applied by axially moving rollers in order metal to flow plastically and distribute the metal along length of the mandrel until precise required thickness is obtained. The process also provides for thicker segment each end to accommodate adequate thickness distribution in the end dome formed by metal spinning operation. The flow forming process requires enormous hydraulic pressures and hardened steel mandrel and rollers. The process tends to be very expensive and capital intensive if the production volume is small.
Welding or brazing of the liner is not permitted for regulated composite cylinders. Once formed, the liner is wrapped with high-strength composite fibers, usually resin-impregnated continuous filaments by means of winding method. Generally, the fibers used are fiberglass, KEVLAR (ARAMID), carbon, graphite, or newly developed basalt fiber. Epoxy or polyester resins are generally used as the matrix material to bind the fibers together and provide structural quality to the composite.
The detailed design and analysis process associated with a high-pressure composite cylinder is performed by iteration of design analysis and prototype testing to establish reliability of composite cylinder performance under various constraints related to fabrication, inspection, maintenance and cost of manufacturing. The choice of the liner design and manufacturing process plays an important role because the cost of a typical load-sharing seamless liner could well exceed 60% of the cost of making a composite cylinder. The design process cost, manufacturing, and Quality Assurance/Quality Control (QA/QC) testing add to the overall cost to make the composite cylinder price cost-prohibitive, particularly when compared with metallic cylinders.
The liner of the invention complies with the latest government regulations and applicable design and performance standards, which require that the liner be seamless. The invention is intended for principal application to composite cylinder designs that must be compliant with one or more of various International and United States regulatory standards.
Generally, the liner of this invention will be compliant with contemporary regulations and standards for the design, manufacturing and use of composite cylinders. Well known examples of such regulations and standards include DOT FRP-1 (Basic Requirements For Fiber Reinforced Plastic (FRP) TYPE 3FC Composite Cylinders) (re: fully overwrapped-Glass Fiber, KEVLAR); DOT FRP-2 (Basic Requirements For Fiber Reinforced Plastic (FRP) Type 3HW Composite Cylinders (re: hoop wrapped-glass, KEVLAR); DOT CFFC (Basic Requirements For Fully Wrapped Carbon-Fiber Reinforced Aluminum Lined Cylinders (DOT-CFFC)); ISO 11119-1: 2002 (Gas cylinders of composite construction—Specification and test methods-Part 1: Hoop wrapped composite gas cylinders); ISO 11119-1: 2002 (Gas cylinders of composite construction—Specification and test methods-Part 1: Hoop wrapped composite gas cylinders); ISO 11119-2: 2002 (Gas cylinders of composite construction—Specification and test methods-Part 2: Fully wrapped fibre reinforced composite gas cylinders with load-sharing metal liners); BS EN 12245: 2002 (Transportable gas cylinders-Fully wrapped composite cylinders).
The liner of this invention has a significantly lower-cost compared to the state of the art liner for principal use in a composite reinforced vessel. In combination with the method of manufacture of the liner, an embodiment of the invention enables the use of lower-cost commercially available extruded metal tube of uniform thickness. A metal end cap for one or both ends (depending on how the cylinder is made) provides adequate thickness and bending stiffness to withstand pressure cycling related stresses and prevent fatigue failure. The metal end cap provides performance at least equivalent to that provided by prior art practice of increased wall thickness in the dome. The prior art practice requires a complex flow forming process, which is a significant cost in making the liner. The metal end cap is a precise mating part that can be slipped on or screwed on to the end dome and secured by without the use of a weld, preferably using a structural adhesive, shrink fit, a lock nut or combination of these. The method of making a liner in accordance with the invention simplifies manufacture and lowers costs of manufacture of a composite reinforced cylinder.